Cellular wireless networks typically comprise user equipments (UE) such as mobile handsets which may communicate via a network interface comprising a radio transceiver to a network of base stations connected to a telecommunications network. Such cellular wireless networks have undergone rapid development through a number of generations of radio access technology. The initial deployment of systems using analogue modulation has been superseded by second generation (2G) digital systems such as GSM (Global System for Mobile communications), implementing GERAN (GSM Enhanced Data rates for GSM Evolution Radio Access Network) radio access networks, and these systems have themselves been replaced by or augmented by third generation (3G) digital systems such as UMTS (Universal Mobile Telecommunications System), implementing the UTRAN (Universal Terrestrial Radio Access Network) radio access networks. Third generation standards provide for a greater throughput of data than is provided by second generation systems; this trend is continued with the introduction of High Speed Packet Access (HSPA), which may augment third generation systems, providing a high capacity packet switched downlink. HSPA typically uses adaptive modulation and coding to provide increased capacity when a channel has a good quality, for example a high signal to noise ratio. In a system such as HSPA using adaptive modulation and coding, a succession of Channel Quality Indicators (CQIs) is typically fed back from a receiver, typically at a user equipment, to a serving node for use in determining a transmission format, which may include a type of modulation and a type of coding, for use on a downlink from the node to the user equipment.
Multiple transmitter schemes, such as MIMO (multiple input, multiple output) and MIXO (multiple input, any output) have been proposed for use with HSTA and other wireless transmission formats. A multiple transmitter scheme may use multiple transmit antennas to provide a number of transmission streams, one or more or all of which may be received at a given user equipment, providing potentially greater capacity than a single transmitter scheme. A transmission stream may correspond to a transmitted beam, and may be referred to as a layer, and beams may overlap spatially. Multiple transmitter schemes may be used as part of a transmission format using adaptive modulation and coding, for example in a HSPA system. The CQI fed back from a user equipment to a serving node may indicate a transmission format preferred by the user equipment, including information relating to a number of multiple transmitter transmission streams on which data intended for the user equipment is to be transmitted. A second type of CQI may also be fed back, indicating a less preferred transmission format as a fallback position, which may be selected by the server for transmission if, for example, only a single transmission stream is to be transmitted.
In multiple transmitter schemes providing more than two transmission streams, there may be multiple potential fallback positions, corresponding to different numbers of transmitted transmission streams. The signalling overhead associated with sending a CQI appropriate to each possible number of transmission streams may be undesirable since it may increase the reporting delay of each individual report or increase overhead hence reducing system capacity for payload data.
It is an object of the invention to address at least some of the limitations of the prior art systems.